Fruits and vegetables that are harvested a long time before they are consumed undergo heavy losses of vitamin C through enzymatic decomposition. For example, the vitamin C content of apples stored under domestic conditions falls to about 1/3 of the original value after only 2-3 months. Green vegetables stored at room temperature lose practically all their vitamin C after only a few days, though losses are lower when stored at 0°C. Newly harvested potatoes have a vitamin C content of about 30 mg/100 g; this figure falls rapidly to about 8 mg/100 g after 8-9 months storage and to virtually nil after another 2 months (Marks, 1975).
Foods are processed in various ways to lengthen the time they can be stored, e.g. by canning, freezing, drying, sterilization and irradiation, and vitamin C losses vary depending on the method used. Before canning or freezing, vegetables are "blanched", i.e. exposed briefly to boiling water or steam to inactivate enzymes which have a detrimental effect during storage. Loss of vitamin C due to blanching is between 13-60%. Short exposure to high temperatures is less harmful than longer heating at lower temperatures. The vitamin C losses during blanching are caused either by oxidation or by leaching; and small pieces of vegetable lose more vitamin C than do large pieces. Losses can be reduced if vegetables are cooked rapidly after blanching. See Figure 2 for the percent of vitamin C retention during the processing and cooking of peas treated in different ways.
Figure 2: Percentage of vitamin C retention during processing and cooking of peas treated in different ways
Source: Marks 1975
Vitamin C loss also occurs during heat sterilization even though oxygen is excluded during this process. Foods stored in cans are relatively stable and losses after storage for 2 years at 10°C amounted to only about 15% of the initial value. Variations are found, however, depending on the degree of enzymatic destruction that has occurred during the blanching and sterilization phase (Marks, 1976).
Freezing can be an effective method of food preservation since vitamin losses due to chemical decomposition are minimal. However, rapid decomposition may occur during thawing, especially in vegetables. Grant and Alburn (1965) have shown that vitamin C oxidation is faster in ice than in water. For example, frozen orange juice concentrates lost about 10% of their vitamin C content during 12 months storage at -23°C. The figures in the literature for vitamin C losses during deep freezing vary considerably, the average being 25% of the initial value.
Freeze drying is probably one of the best preservation methods as far as vitamin retention is concerned, although it is not widely practised. Hot air drying of vegetables results in a 10-15% loss of vitamin C under the most favourable conditions. The commercial dehydration of potatoes can cause losses of 35-45% (Berry Ottaway, 1993).
The stability of vitamin C in soft drinks and fruit juices varies widely according to the composition and oxygen content of the solution. It is very unstable in apple juice but quite stable in blackcurrant juice, possibly as a result of the protective effects of phenolic substances with antioxidant properties. Light can have an impact on the stability of vitamin C in milk with the extent of losses being dependent on the type of container in which the milk is stored and the length and conditions of exposure. Bottled orange drinks exposed to light have been found to lose up to 35% of their vitamin C within 3 months.Vitamin C losses in milk are around 25% during pasteurisation and about 60% during sterilization. Up to 100% of vitamin C content is lost in UHT (ultra-high temperature, short time treated) milk stored for 3 months (Berry Ottaway, 1993).
Grain milling to produce white flour minimizes the content of bran and germ and consequently vitamin C. White flour retains half the amount of vitamin C found in whole wheat flour.